End-Tidal Gas Analysis

Describe the principles of pulse and tissue oximetry, co-oximetry and capnography, including calibration, sources of errors and limitations

Principles

Several mechanisms for E_TCO₂ measurement exist:

• Infrared Spectroscopy
• Colourimetric Methods
• Rayman Scattering
• Gas Chromatography

Infrared Spectroscopy

Infrared spectroscopy relies on the fact that:

• Gases with two or more different atoms will absorb infrared radiation
• Different gases absorb different wavelengths to different degrees
• Measuring the absorbed wavelengths and comparing with the likely composition of a mixture, a system can be designed using a specific wavelength to measure gas concentrations and avoid interference

End-tidal gas analysis using infrared light is used in the measurement of:

• CO₂
  Capnography is the continuous measurement and graphical display of the partial pressure of CO₂ in expired gas. This is the most common method to measure E_TCO₂.
• Anaesthetic agents

Measurement of CO₂

Components:

• Sapphire sampling chamber containing gas sample
  • CO₂ absorbs infrared radiation at a peak wavelength of 4.28μm
  • The sapphire lens only allows 4.28μm light through
• Emitter
• Detector
• Microprocessor
• Display

Method:

• Light is emitted and passes through the sampling chamber
  A lens is used to focus emitted light.
• Levels of radiation are measured on the other side of the chamber
• Levels correspond to the amount of gas present in the sample
• The less radiation that reaches the detector, the more gas there is in the sample absorbing it

Equipment Errors

Errors can be classified into:

• Specific to technique
  • The collision broadening effect
    Intermolecular forces vary depending on their proximity to other molecules in the gas mixture. A change in intermolecular forces may alter their bond-energy and the frequencies at which they absorb radiation. It can be overcome by:
    • Correcting for the presence of other gases
    • Manually adjusting the obtained values
  • Crossover with other gas mixtures
    CO₂ and N₂O have similar absorbance spectra, and may lead to error when a device is not designed to measure both wavelengths.

• Failure of equipment
  These can be overcome by use of double-beam capnometer. This uses a reference chamber which contains CO₂-free air, and the same emitter-detector system. All absorption from this system must occur due to artefact (as no CO₂ is present). The artefactual component is then subtracted from the value detected in the main chamber. This corrects for:
  • Variable amount of infrared radiation released
  • Variable sensitivity of the detector
  • Variable efficacy of the crystal window and lens system

• Relating to type of capnometer used
  E_TCO₂ may be either side-stream or in-line.
  • Side-stream CO₂ involves a length of narrow tubing drawing gas from the expiratory limb of the breathing circuit (typically from the HME filter) to the capnograph
    • Side-stream requires a flow of 150 ml.min^-1
    • Has a (pretty insignificant) delay (<1s) in measurement
    • May be blocked by water vapour, and require use of a water trap to remove condensation
  • In-line systems have a sampling chamber attached in-line with the ETT
    • The sampling chamber slightly increases the dead-space of the circuit
      May be relevant in children or very difficult to ventilate patients.
    • Adds weight to patient end of the breathing circuit
    • Require heating to 41°C to avoid condensation

Normal E_TCO₂ Waveform

The normal trace consists of four components:

1. The baseline
   This consists of:
   • Inspiratory time
   • Early dead-space exhalation
     This is the period immediately before phase 2, where some gas with a PCO₂ of 0 is exhaled.
2. Alveolar exhalation, where PCO₂ rises rapidly
3. Alveolar plateau, where PCO₂ flattens
   The highest-point of this curve is labeled E_TCO₂.
4. Inspiration, where PCO₂ returns to 0

E_TCO₂ Waveform Variations

Airway obstruction:

• Occurs due to uneven emptying of alveoli with different time-constants

Hyperventilation:

• Lower E_TCO₂ with shorter baseline
• Plateau phase may not occur at very high respiratory rates

Rebreathing:

• Baseline increases as inspired CO₂ is measured from gas analyser

Changes in E_TCO₂

Normal E_TCO₂ is 32-42 mmHg, whilst normal PaCO₂ is 35-45 mmHg.

High E_TCO₂

This may be from:

• Decreased ventilation
  • Decreased RR
  • Decreased V_T
  • Increased V_D and therefore a greater V_D:V_T ratio
• Increased production of CO₂
  • Increased metabolic rate
    • Sepsis
  • Tourniquet release
  • ROSC following arrest
• Increased inspired
  • Rebreathing (i.e. equipment/ventilator malfunction)
  • External source of added CO₂

Low E_TCO₂

Rapid Loss of E_TCO₂

• Failure of ventilation
  • Circuit disconnect
  • Airway obstruction
  • Bronchospasm
• Failure of circulation
  • Cardiac arrest
  • Shock

Gradual Loss of E_TCO₂

• Increased V_A (i.e. increased MV)
• Decreased CO₂ production
  • Hypometabolic state
    • Hypothermia
• Increased V_D, i.e. V/Q mismatch
  • Increased West Zone I physiology:
    • Hypotension
    • Increased RV Afterload:
      • PE
      • High PEEP
• Sampling error
  • Air entrainment into the sample chamber
  • Inadequate V_T

Discrepancy between E_TCO₂, PACO₂, and PaCO₂

The normal gradient between PaCO₂ and E_TCO₂ is 0-5 mmHg. Healthy and awake individuals should have essentially no (<1ml) alveolar dead space, and so essentially no gradient. This gradient is increased in patients with:

• V/Q mismatch
  • E_TCO₂ will underestimate arterial CO₂ as gas from un-perfused alveoli (with negligible CO₂) will dilute CO₂ expired gas

Colourimetric Methods

Litmus paper which changes colour when exposed to hydrogen ions (produced by CO₂) can be used to confirm endo-tracheal intubation, though they may generate false-positive results due to gastric pH.

References

1. Cross ME, Plunkett EVE. Physics, Pharmacology, and Physiology for Anaesthetists: Key Concepts for the FRCA. 2nd Ed. Cambridge University Press. 2014.
2. Davis PD, Kenny D. Basic Physics and Measurement in Anaesthesia. 5th Ed. Elsevier. 2003.
3. Leslie RA, Johnson EK, Goodwin APL. Dr Podcast Scripts for the Primary FRCA. Cambridge University Press. 2011.